| As one of important structural materials, titanium and titanium alloys are widely used in aerospace, petro-chemical and biomedical industries because of their outstanding corrosion resistance properties, high specific strength and excellent biocompatibility. Mo is one of the elements which have been judged to be main chemical composition and constituent modification in titanium alloys design, and it has been selected as one of the important alloying elements to develop high temperature, high strength and corrosion resistance titanium alloys. However, the strengthening effects and ductility improvement behavior of Mo in titanium alloy are lack of systematic study. In present study, the phase structure constitution, microstructure evolution, beta grain growth behavior, strengthening mechanism and plastic deformation behavior in simplified model titanium alloy with different Mo contents are systematically and deeply analyzed, the microstructure and strengthening effect of Ti-1300 titanium alloy with high Mo equivalent have been also investigated for proving trial, and provide theoretical basis for the design and development of new high performance titanium alloy.Thermodynamic analysis validated that Mo was functioned as a β-phase isomorphous stabilizers in titanium alloy. The microstructures of Ti-1Mo, Ti-2Mo and Ti-4Mo alloys after machining were composed of a mixture of lamellar β-phase and α matrix. Mo element was main distributed in lamellar β-phase, and lamellar β-phase grew with the increasing of Mo contents and distributed in certain crystal orientation. Ti-15Mo and Ti-20Mo alloys were mainly contained equiaxed β phase with the average grain size range between 50～80μm. The microhardness of Ti-xMo alloys was increased with the enhanced interaction between Mo atom and dislocation due to increasing of Mo contents.Grain growth behavior of Ti-xMo alloys had been studied. Ti-1Mo and Ti-2Mo alloys mainly contained hexagonal a phase, while Ti-4Mo alloy had some acicular a’ grains precipitated in β grains. When Mo content increased to 10 wt% or higher, the retained β phase became the only phase. There was an exponential relationship between β-grain size and solution time, and the β-grain size also grew with the increasing of solution temperature in certain time condition. Because of being free from the "solute drag effect", the activation energy for grain growth of Ti-4Mo alloy (83.30 kJ/mol) was also lower than that of Ti-20Mo alloy (272.16 kJ/mol), but the time exponent of Ti-4Mo alloy (0.42) was higher than that of Ti-20Mo alloy (0.26) due to the low dislocation density. The strength properties of Ti-xMo alloys increased with the increasing of Mo content and decreasing of grain size, the relationship of grain size and strengthening effects coincided with the Hall-Petch mechanism. Solution strengthening, grain refinement effect and phase transition were the mainly strengthening mechanisms of Mo in titanium alloy.The microstructure and mechanical properties of Ti-6Al based alloys were investigated. The Ti-6Aland Ti-6Al-1Mo were composed of hexagonal a phase and α2 phase, but the hexagonal a phase morphology displayed from feather-like shape (Ti-6A1) to coarse lath type structure (Ti-6Al-1Mo). The Ti-6Al-3Mo and Ti-6Al-5Mo alloys were dominated by elongated, equiaxed a phase, and few α2 phase and the retained β phase, but the grain size of a phase in Ti-6Al-5Mo alloy was much smaller than those in Ti-6Al-3Mo alloy. The lattice parameters of a phase changed little with the Mo addition:a increased and c decreased. Ti-6Al-3Mo and Ti-6Al-5Mo exhibited superior comprehensive mechanical properties.Plastic deformation behavior of Ti-xMo and Ti-6Al-xMo alloys had been evaluated. The microstructures of Ti-4Mo, Ti-6Al-3Mo and Ti-6Al-5Mo alloys after deformation were composed of equiaxed β-grain and a phase, but the morphology of intracrystalline hexagonal a phase became the acicular shape from lath (before deformation). The β-grain of Ti-20Mo alloy after deformation was elongated into fibrous shape due to stress concentration, and some subgrain formed in P-grain, the deformation mode was supported by slip. The tensile strength and elongation of Ti-20Mo was 1011.30MPa and 20.5%, respectively. The highest tensile strength was appeared in Ti-6Al-5Mo through the higher work hardening rate, but the plastic became badly. The fractured surfaces of Ti-4Mo, Ti-6Al-3Mo and Ti-6Al-5Mo had a characteristic intercrystalline of cracking with a large number of tear ridges around cleavage plane, but Ti-20Mo was more of transgranular cleavage cracking in conjunction with large dimples and displayed ductile fracture.The effects of high temperature thermomechanical treatment and heat strengthening processing on microstructure and mechanical properties of Ti-1300 alloy with high Mo equivalent were investigated. The microstructure of Ti-1300 alloy after high temperature deformation in α+β temperature range was consisted of spheric a phase and equiaxed P phase, The microstructure and grain of Ti-1300 alloy with large deformation can be strongly refined, and became homogeneously. After solution or quenching treatment, αp phase with strip shape distributed in grain boundary and αs precipitated and dispersed in β matrix. The mechanical properties of the alloy after solution or quenching treatment was higher than the alloy after high temperature deformation, this was related to ap phase with small size and dispersion precipitated as phase. Dispersion precipitation was the mainly strengthening mechanisms of Ti-1300 alloy under high temperature thermomechanical treatment. Ti-1300 alloy was possessed of an excellent workability, which attributed to the elongation of the solution treated alloy above 20% and area reduction over 60%. When the alloy was under different aging conditions in α/β solution treatment, a relative high strength level above 1350 Mpa with excellent elongation above 12% was obtained, and the microstructure composed of a mixture of αp phase, as phase and β phase. The width of ap grew with solution temperature, and the as phase precipitated and distributed in β matrix. When the alloy was under aging conditions in β solution treatment, the microstructure composed of an acicular shape αs phase and β matrix. And the alloy exhibited the super strength around 1640 MPa, but the ductility became badly, this phenomenon can be explained by the fine non-homogeneous distributed as phase. The optimum heat processing parameter was obtained:the deformation temperature was in the range of 750-790℃ and the deformation was above 90%. The optimum heat treatment was also acquired:the solution temperature was between 750～800℃ and the aging temperature was in the range of 500～560℃. |
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